1. Field of the Invention
The present invention relates to an ohmic electrode with high heat resistance and low contact resistance, formed on a p-type diamond semiconductor and a method for forming it.
2. Related Background Art
Diamond has a large bandgap of about 5.5 eV and thus has no temperature range corresponding to the intrinsic region below 1400.degree. C. It is, therefore, excellent in heat resistance. Also, diamond has the thermal conductivity of 20 W/cm.multidot.K, which is ten times larger than that of silicon. Thus, it is also excellent in heat radiation. Because of such properties, semiconductor devices made of diamond can be operated at high temperature and can be improved in the degree of integration of circuits.
These days, p-type diamond semiconductors formed by doping diamond with boron are expected to be applied to light-emitting devices to emit light in the range of various wavelengths from ultraviolet light to visible light, environmentally resistant semiconductor devices required to have high heat resistance and high thermal conductivity, and etc. In order to produce such semiconductor devices, ohmic electrodes formed on the p-type diamond semiconductor need to be provided with high heat resistance and low contact resistance.
A conventional ohmic electrode is formed by successively building a contact electrode layer of Ti, a diffusion barrier layer of Mo, and a lead electrode layer of Au up on a p-type diamond semiconductor. Ti forming the contact electrode layer is a metal having a relatively high melting point, but has a resistivity of higher than materials such as Au, Al, etc. Thus, Au is layered as a lead electrode layer above the contact electrode layer so as to reduce lead wire resistance in order to improve device performance.
However, when the lamination structure of Ti and Au is subjected to heat treatment at a temperature of about 300.degree. C., Ti forming the contact electrode layer readily diffuses into Au forming the lead electrode layer to precipitate over the surface of the lead electrode layer, thus increasing the lead wire resistance. Even in use at room temperature current supply to such a semiconductor device causes Joule heat generated to induce mutual diffusion of Ti and Au, which results in precipitation of Ti over the surface of the lead electrode layer. To prevent the diffusion of Ti, Mo is interposed as a diffusion barrier layer between the contact electrode layer and the lead electrode layer.
The above prior art technology is described in detail in Japanese Laid-open Patent Applications No. 1-246867 and No. 2-260470.
In the above conventional ohmic electrode the thickness of Ti forming the contact electrode layer is, however, relatively large, i.e., about 300 to 500 nm. Then, the interposition of Mo as the diffusion barrier layer between the contact electrode layer and the lead electrode layer cannot fully prevent Ti from diffusing from the contact electrode layer toward the lead electrode layer. As a result, Ti precipitates over the surface of the lead electrode layer of Au, which raised a problem of degrading the device performance because of an increase in lead wire resistance.
The present invention has been accomplished to solve the above problem. It is, therefore, an object of the present invention to provide an ohmic electrode formed on a p-type diamond semiconductor, which can improve the device performance, based on reduced lead wire resistance, and a method for forming the ohmic electrode.